In the plane-only mode, we try to place every view on a hardware plane,
and fail if we can't do this. This requires a full walk of the scene
graph to come up with a complete configuration in order to be able to
test.

In mixed mode, we know at least some visible views will fail to be
promoted to planes and must be composited via the renderer. In order to
still use some planes where possible, we use atomic modesetting's
test-only mode to incrementally test configurations.

We know that the renderer output will always be visible, and because it
is the renderer, that it will be occupying the scanout plane underneath
everything else. The actual renderer buffer doesn't materialise until
after assign_planes, because it cannot know what to render until then.

However, in order to test whether a configuration is valid, we need the
renderer buffer in the scanout plane. For testing, we fake this by
temporarily stealing the old buffer - if it seems sufficiently
compatible - and placing it in the state we construct. This is used to
test whether or not a renderer buffer will work with the addition of
overlay planes.

Doing this incremental testing will allow us to enable plane usage for
atomic by default, since we know ahead of time that our chosen plane
configuration will work.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>